I just happen to own a Gen -3 Viper as well as a Turbo Small Block Dakota.

First off $12,000 for ANY Twin Turbo set-up pre-fabbed is not a bad price for any vehicle. I had almost that in just parts for the twins for my Dakota and still had to have a few pieces fabbed. Was quoted $15,000 to have one built from scratch about 6 yrs ago.

My Viper on the other hand makes 500hp/490trq from the factory. Idles at just over 750rpms and gets 90% of that Trq by 2200rpms and carries it all the way to redline. This is all from a 9.6:1 compression motor.
The car has 3.08 gears and will do 80+mph in second gear. If you take it over 2500rpms YOU better have it pointed in the right direction and have you marbles together. IT is a Handful. Gets about 23mpg on the Highway.

Mopar has an Upgrade kit they claim brings it up to the Gen-4 HP spec of 640hp BUT from people that have actually installed it and had it dyno'd it is closer to the New 2013 Viper Specs at about 680hp. All in a N/A Pump Gas engine that idles like a Kitten and drives like a beast. And I can get that kit for about $7000 vs the Mopar Price of $10000 if I join VCoA for $150/yr

At the end of the day it's all about the CUI. 488 in the Gen 1&2 , 505 in the Gen-3 and 512 in the Gen-4. 2103=????

The sound is like a P's'd off 6-cylinder to about 2000rpms then it has that distinct sound that let's you know it's a SupeCar.

All that said!!! Yes I see why people choose these engine to transplant. And like an R3, Hemi, or any other top performer the prices are about the same for the same level of performance AND if you know where to look you can save yourself a lot of $$$$.